The present invention relates to the pattern structure of a printed circuit board for mounting various kinds of electronic chip parts.
It has been customary to solder electronic chip parts to patterns provided on a printed circuit board and implemented by, for example, copper foil. In the SMT (Surface Mounting Technology) art, the above-mentioned patterns are referred to as pads. In many cases, the pads are provided with a rectangular shape although the shape depends on the configuration of the portions of chip parts to be soldered. The pads located in a circuit connecting portion are often implemented as narrow patterns while the pads located in a ground circuit portion are often implemented as a broad continuous pattern, i.e., solid pattern. Generally, a printed circuit board is provided with a solder resist coating on the surface thereof to prevent solder from depositing in unexpected portions or from bridging. In the circuit connecting portion, the pad is spaced apart from the solder resist coating by a gap. In the ground circuit portion, the pad is formed by part of a solder resist coating provided on a solid ground pattern for a grounding purpose.
The problem with the above-described conventional pattern structure is that the pad in the circuit connecting portion and the pad in the ground circuit portion are often different in size or area from each other. When a chip part is soldered to such pads by, for example, a fellow process, the temperature elevation rate differs from one pad to the other pad since the pattern area, i.e. , the area of copper foil greatly differs. The difference in temperature elevation rate results in a difference in the melting time of solder and, therefore, a difference in surface tension. This aggravates the probability of a so-called tombstone phenomenon wherein the chip stands. Furthermore, since the two pads are different in area from each other and since the amount of solder is fixed, the spread of solder and, therefore, the configuration of the resulting fillet differs from one pad to the other pad, making the quality check troublesome.